Preignition Damage; Reversed Coil Polarity; Splashed Deposits - Mercury MerCruiser Service Manual

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Preignition Damage

Preignition damage is caused by excessive high temperatures. The center electrode melts first, followed by the ground

electrode. Normally, the insulators are white, but may be dirty if the plug has been misfiring. Check for the correct plug heat

range, incorrect rotor location, lean fuel mixture, incorrect fuel used, malfunctioning cooling system, leaking intake manifold, or

lack of lubrication. Replace the spark plugs.

Concave erosion of the ground electrode is an indication of reversed polarity. The center electrode will show only normal wear.

The engine will misfire and idle rough. To correct, reverse the primary coil leads. Replace the spark plugs.

Splashed deposits, which sometimes occur after long delayed tune‑up, accumulate after a long period of misfiring. When

normal combustion temperatures are restored, upon the installation of the new plugs, the deposits loosen from the top of the

piston and the cylinder head and are thrown against the hot insulator. Service (clean, file, and regap) plugs and reinstall.

90-8M0099748 eng DECEMBER 2015

Engine Troubleshooting

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